The native fluorescence of human and mouse skin has been shown to vary with aging and UV exposure in a predictable manner. See Brancaleon et al., J. Invest. Dermatol. 113(6):977-982, 1999; Kollias et al., J. Invest. Dermatol.111(5):776-780, 1998; Leffell et al., Arch Dermatol. 124(10):1514-1518, 1988; Na et al., J. Invest. Dermatol. 116(4):536-540, 2001; and Tian et al., J. Invest. Dermatol. 116(6):840-845, 2001. Thus, fluorescence spectroscopy has been proven to be an objective quantitative method for studying skin aging and photoaging.
The major fluorescence bands that have been detected by in vivo fluorescence spectroscopy include: a) a band assigned to tryptophan (maximum at 295 nm excitation, 345 nm emission), b) a band assigned to pepsin digestible collagen cross-links (335 nm excitation, 390 nm emission), c) a band assigned to collagenase digestible collagen cross-links (370 nm excitation, 460 nm emission), and d) a band most likely due to elastin and collagen cross-links (390-420 nm broad band excitation, 500 nm emission). See Gillies et al. J. Invest. Dermatol, 115(4):704-707, 2000. Secondary fluorescence bands have been identified that may be related to collagen peroxidation (Odetti et al. Lab Invest. 70(1):61-67, 1994) or elastin (Leffell et al., Arch Dermatol., 124(10):1514-1518, 1988): one at 356 nm excitation, 420 nm emission and another at 390 nm excitation, 460 nm emission respectively.
The fluorescence signal assigned to tryptophan moieties measured in situ was found to increase when epidermal proliferation increases. See Kollias et al., J. Invest. Dermatol, 111(5):776-780,1998 and Zhang et al., Lasers Surg. Med. 20(3):319-331, 1997. This was verified by inducing epidermal repair after mechanical insult, e.g. tape stripping. See Brancaleon et al., J. Invest. Dermatol. 113(6):977-982, 1999. Furthermore, α-hydroxy-acid-induced increases of cellular turnover in human epidermis caused the 295 nm excitation band to increase in a dose dependent manner. See Doukas et al., Photochem. Photobiol. 74(1):96-102, 2001. In SKH hairless mice the fluorescence due to tryptophan moieties decreases with age, implying an age-related reduction of the epidermal cell turnover rate. See Kollias et al., J. Invest. Dermatol. 111(5):776-780,-1998.
Non-enzymatic glycosilation of proteins occurs spontaneously with aging (See Monnier et al., Clin Endocrinol Metab 11(2):431-452, 1982; Njoroge et al., J. Biol. Chem. 263(22):10646-10652, 1988; Sell et al., J. Biol Chem 264(36):21597-21602, 1989; and Shaklai et al., J. Biol Chem 259(6):3812-3817, 1984) resulting in increased protein absorbance and fluorescence (Maillard reaction). The glucose-protein adduct rearranges and dehydrates to form brown and fluorescent pigments, which may form cross-links resulting in decreased protein solubility and altered mechanical properties. Such cross-links are evident in long-lived proteins, such as elastin and collagen. The accumulation of fluorescing cross-links in collagen has been used as a marker for the observed accelerated rate of aging in diabetes. See Monnier et al., Clin. Endocrinol. Metab 11(2):431-452, 1982. In SKH mice the magnitude of the pepsin digestible collagen cross-link fluorescence maximum increases with chronological aging, whereas the increase in the magnitude of the collagenase digestible collagen cross-link and the elastin-associated fluorescence maxima is modest. See Kollias et al., J. Invest. Dermatol. 111(5):776-780, 1998. Similar trends have been observed in rats ex vivo (Odetti et al., Lab Invest. 70(1):61-67, 1994), in human buttock skin in vivo (Na et al., J. Invest. Dermatol 116(4):536-540, 2001), and in ex vivo human dermis taken from skin around the operating area of patients undergoing vascular surgery (Odetti et al., Metabolism 41(6)655-658, 1992).
Applicants have surprisingly found that skin native autofluorescence is a tool to evaluate skin health and the effects of aging (e.g., chronological aging as well as photoaging) on skin health.